Fast scanning micromirrors and microlenses with large vertical displacement (LVD) ranges are desired in a variety of applications, such as wave-front shaping in adaptive optics, interferometry systems, and spatial light modulators. Recently, endoscopic optical coherence tomography (OCT), optical coherence microscopy (OCM), nonlinear optical imaging, and confocal microscopy are emerging as powerful biomedical imaging modalities. These imaging modalities typically require hundreds of microns of vertical displacement for platforms incorporated with the micromirrors and/or microlenses. In OCM and confocal microscopy imaging systems, the vertical scanning ranges of the microlens's focal plane should be 200 μm up to about 2 mm, for example. Linear translating retroreflectors can be dispersion-free and polarization-insensitive, but are typically bulky and slow. The Fourier domain rapid scanning optical delay line (RSOD) can achieve several kHz scans, but it is complex and lossy and requires dispersion compensation. Most existing OCT systems perform image scanning of samples by moving an OCT probe or rotating the distal end of an optical fiber, which is slow and has non-uniform optical coupling.
Currently, two main actuation mechanisms are used to obtain large vertical displacement of the micromirrors and microlenses for OCT, OCM, and confocal microscopy. The first is electrostatic actuation, which can only provide tens of microns of scanning range. The driving voltages required are usually hundreds of volts; thus, such actuation is generally not suitable for biomedical applications. The second is electrothermal actuation, which is capable of hundreds of microns vertical scanning at low driving voltages. These electrothermal actuators conventionally have two sets of complementarily-oriented thermal bimorph beams, which have as a goal to keep the mirror plate or lens holder parallel to the substrate and take advantage of the large vertical displacement at the tip of the long rotational beams.
However, the designs using electrothermal actuation to date have had a serious lateral shift problem. As an example, a particular LVD microlens used for confocal microscopy can generate a maximum 0.71 mm vertical displacement, but has a lateral shift as large as 0.42 mm Such large lateral shifts tend to greatly distort the microscopic image. The lateral shift in LVD micromirrors also significantly reduces the effective optical aperture size. Another disadvantage of the previous LVD designs using electrothermal actuation is that a certain ratio of the driving voltages for the two complementary actuators needs to be maintained in order to obtain a purely vertical motion during the actuation.
Thus, a need remains for a microactuator that is capable of providing a large vertical displacement with reduced tilting and/or reduced lateral shift.